Chemical vapor deposition and infiltration (CVD/CVI) is a well-known process for depositing a binding matrix within a porous structure. The term “chemical vapor deposition” (CVD) generally implies deposition of a surface coating, but the term is also used to refer to infiltration and deposition of a matrix within a porous structure. The term CVD/CVI is intended to refer to infiltration and deposition of a matrix within a porous structure.
CVD/CVI is particularly suitable for fabricating high temperature structural composites by depositing a carbonaceous or ceramic matrix within a carbonaceous or ceramic porous structure resulting in very useful structures such as carbon/carbon aircraft brake disks, and ceramic combustor or turbine components. The generally known CVD/CVI processes may be classified into four general categories: isothermal, thermal gradient, pressure gradient, and pulsed flow. The specific process times and steps, reactant gas and CVD/CVI furnaces and associated apparatus may vary depending on which of these four general categories is utilized.
U.S. Pat. No. 6,162,298 and corresponding European Patent Application EP 0 997 553 A1 to Rudolph describe these and other CVD/CVI processes and apparatus in further detail. Rudolph particularly describes a sealed reactant gas inlet for a CVD/CVI furnace.
FIG. 1 is a side cross-sectional view of a furnace according to U.S. Pat. No. 6,162,298. A generally cylindrical furnace 10 configured to be employed with a high temperature process is shown. The furnace includes a steel shell 12 and a steel lid 14. The shell 12 includes a flange 16 and the lid 14 includes a mating flange 18 that seals against flange 16 when the lid 14 is installed upon the shell 12, as shown in FIG. 1. The lid also includes a vacuum port 20.
The shell 12 and lid 14 together define a furnace volume 22 that is reduced to vacuum pressure by a steam vacuum generator (not shown) in fluid communication with the vacuum port 20. The shell 12 rests upon a multitude of legs 62. The furnace 10 also includes a cylindrical induction coil 24 adjacent a cylindrical susceptor 26. The induction coil 24 includes coiled conductors 23 encapsulated by electrical insulation 27.
During operation, the induction coil 24 develops an electromagnetic field that couples with the susceptor 26 and generates heat within the susceptor 26. The induction coil 24 may be cooled, typically by integral water passages 25 within the coil 24. The susceptor 26 rests upon a susceptor floor 28 and is covered by a susceptor lid 30. A cylindrical insulation wall 32 is disposed in between the susceptor 26 and the induction coil 24. A lid insulation layer 34 and a floor insulation layer 36 are disposed over the susceptor lid 30 and beneath the susceptor floor 28, respectively.
The susceptor floor 28 rests upon the insulation layer 36, which, in turn, rests upon a furnace floor 38. The furnace floor 38 is attached to the shell 12 by a floor support structure 40 that includes a multitude of vertical web structures 42.
A reactant gas is supplied to the furnace 10 by a main gas supply line 44. A plurality of individual gas supply lines 46 are connected in fluid communication with a plurality of gas ports 48 that pass through the furnace shell 12. A plurality of flexible gas supply lines 50 are connected in fluid communication with the gas ports 48 and a multitude of gas inlets 52 that pass through holes 54 in the furnace floor 38, the floor insulation layer 36, and the susceptor floor 28.
U.S. Pat. No. 6,162,298 further describes a gas preheater 56 resting on the susceptor floor 28 and including a multitude of stacked perforated plates 58 that are spaced from other by a spacing structure 60. Each plate 58 is provided with an array of perforations that are horizontally shifted from the array of perforations of the adjacent plate 58. This causes the reactant gas to pass back and forth through the plates, which diffuses the reactant gas within the preheater 56 and increases convective heat transfer to the gas from the perforated plates 58. A multitude of porous substrates 62, for example brake disks, are stacked within the furnace 10 inside the susceptor 26 on fixtures (not shown).
Further, U.S. Pat. No. 6,162,298 is directed toward preventing gas leakage around the gas inlet 52 extending through the hole 54 in the susceptor floor 28 in the CVD/CVI furnace 10. The method and apparatus seal the gas inlet 52 to the susceptor floor 28 with sufficient intimacy to block leakage of gas through the hole 54 around the gas inlet 52 while allowing the gas inlet 52 to cyclically translate through the hole 54, as indicated by arrow 55, due to thermal expansion and contraction induced by thermal cycles in the CVD/CVI furnace 10.
Reactant gas entry through the gas inlets 52 is diffused within the preheater 56 and eventually reaches the porous substrates 62. However, reactant gas leaving the gas inlets 52 follows a tortuous path as it travels back and forth through the plates 58.